Hi DC42 and thanks again.
I measured the phase lag to be about 1.32ms between the current phase and the voltage phase using my oscilloscope. However, that is using a light, unitary PF load (i.e. 40W incandescent lamp). Under higher load (i.e. 1340W unitary PF toaster), the phase shifts decreases to 0.7ms. As mentioned over in the science section, these results have baffled me somewhat. Pito suggested this may have to do with the mains, something I need to follow up on by testing at the 400A power panel vs. in the kitchen.
Either way, either lag cannot be fixed using just the phase register in the MCP3911 since the lag is well outside the realm of the ADC phase register range (+/- 0.3ms with the current settings). As I see it, I have two options: adjust capacitors and resistors as you suggest (not that easy on a 0805 footprint) or accept the lag and simply use offset pairs of readings in the calculations (the ADC is sampling at about 1418 sps).
If my oscilloscope readings are correct, it might make sense to simply pair ampere reading (x-2) with voltage reading (x) to achieve a ~1.4ms correction and then use the phase register to correct the remaining 0.08ms. That is, assuming the light load phase lag is correct.